Polymeric　carbon　nitride　prepared　by　thermal　condensation　of　cyanuric　chloride　with　melamine　was　post-treated　under　vacuum　conditions　at　different　temperatures　in　order　to　study　in　depth　the　structure-performance　relationship.　The　structure,　composition,　photoelectric　and　photocatalytic　properties　of　the　resulting　samples　were　characterized　in　detail　by　physicochemical　means,　such　as　X-ray　diffraction,　thermogravimetry,　elemental　analysis,　Fourier　transform　infrared　spectroscopy,　scanning　electron　microscopy,　X-ray　photoelectron　spectroscopy,　UV-vis　diffuse　reflectance　spectroscopy,　photocurrent　response,　electrochemical　impedance　spectroscopy　photocatalytic　hydrogen　production,　etc.　The　results　revealed　that　the　untreated　polymeric　carbon　nitride　was　not　a　single　phase　but　a　mixture　consisting　of　unequal-sized　particles　with　different　degrees　of　polymerization　and　chemical　structures　containing　both　s-triazine　and　tri-s-triazine　ring　building　blocks.　This　makes　it　possible　to　modify　polymeric　carbon　nitride　through　a　post-treatment.　Tremendous　changes　in　the　C/N　ratio,　IR　spectra,　photoresponse,　morphology,　and　photocatalytic　activity　occurred　mainly　in　two　temperature　ranges　of　300-500　degrees　C　and　500-600　degrees　C.　Among　all　samples,　the　carbon　nitride　treated　at　500　degrees　C　showed　the　highest　photocatalytic　activity　for　production　of　hydrogen　from　water,　as　a　result　of　the　higher　content　of　the　tri-s-triazine　phase,　better　lamellar　morphology,　wider　photoabsorption,　and　smaller　electrochemical　impedance.　The　vacuum　heat-treatment　at　temperature　above　500　degrees　C　gave　rise　to　the　broken　structure,　and　consequently　the　photoactivity　was　reduced.